Air coil and method of making the same

ABSTRACT

In one aspect, the present invention provides an air coil 20. The air coil 20 may be made from a strip of electrically conductive material 22 which has an insulating material overlying it. For example, the conductive material may comprise copper. The strip 22 is wound into a coil to perform a plurality of windings. The insulating material electrically insulates each of the winding from adjacent ones of the windings.

CROSS REFERENCE TO RELATED APPLICATIONS

The following U.S. patent and commonly assigned applications are herebyincorporated herein by reference:

    ______________________________________                                        Pat.      Effective                                                           No.       Filing Date                                                                              Issue Date TI Case No.                                   ______________________________________                                        5,053,774 07/08/88   10/01/91   TI-12797A                                     5,450,088 11/25/92   09/12/95   TI-16688                                      5,408,243 01/14/93   04/18/95   TI-16561                                      5,471,212 04/26/94   11/28/95   TI-18205                                      ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to identification systems and morespecifically to an air coil antenna and a method for making the same.

BACKGROUND OF THE INVENTION

There is a great need for devices or apparatuses which make it possibleto identify or detect objects over a certain distance without makingcontact. In addition, a need exists to be able to change the data storedin, or operating characteristics of, these devices or apparatuses (e.g.,"program" the devices or apparatuses).

It is, for example, desirable to request, over a certain distance,identifications which are uniquely assigned to an object. Theseidentifications could be stored in the device or apparatus so that, forexample, the object may be identified. A determination may also be madeas to whether or not a particular object exists within a given readingrange.

As another example, physical parameters such as temperature or pressurecan be interrogated directly even when direct contact to the object isnot possible. A device or apparatus of the type desired can, forexample, be attached to an animal which can then always be identified atan interrogation point without direct contact.

There is also a need for a device which, when carried by a person,permits access checking whereby only persons whose responder unitreturns certain identification data to the interrogation unit areallowed access to a specific area. In this case the safeguarding of thedata transfer is a very essential factor in the production of suchdevices.

A further example of a case in which such a device is needed is thecomputer controlled industrial production in which, without theintervention of operating personnel, components are taken from a store,transported to a production location and there assembled to give afinished product. In this case a device is required which can beattached to the individual components so that the components can bespecifically detected in the spares store and taken therefrom.

Several transponder arrangements have been developed. One suchtransponder arrangement is described in U.S. Pat. No. 5,053,774 issuedon Oct. 1, 1991, incorporated herein by reference. This patent describesa transponder unit which has a low energy requirement and does not needits own power source.

Each of these systems, as well as others, requires an antenna for eachof the interrogation unit and transponder to transmit and receivesignals. Therefore, there is a need for antennas with a high qualityfactor (Q), light weight, and manufacturable at a low cost.

One air coil 10 which has previously been used as an antenna isillustrated in FIG. 1. The air coil 10 is formed from a cylindrical wire12 which is wound around a core 14, typically an air gap. The prior anair coil 10, however, has a number of problems. For example, these aircoils have a lower Q than is desirable. In addition, smaller and lighterantennas are desired.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an air coil. In oneapplication, this air coil can be used as an antenna for a transponderor interrogation unit as described in the '774 patent or in othersystems such as the ones described in application Ser. No. 07/981,635 or08/233,839, both of which are incorporated herein by reference. In afirst embodiment, the air coil is made from a strip of electricallyconductive material and an insulating material which overlies the strip.The strip is wound in a coil to form a plurality of windings. Theinsulating material electrically insulates each of the windings fromadjacent ones of the windings.

The air coil antenna of the present invention is particularly usefulwith applications which require light, compact antennas but also needhigh performance. One such application is security badges. The securitybadge could include a card with an air coil of the present inventionincorporated into it. A transponder would then be electrically coupledto the antenna and could be physically attached to the card.

An antenna of the present invention provides an advantage over otherdevices because of the high quality factor or Q. In this context, the Qis defined as a ratio between the imaginary and real components of theimpedance. Since the antenna windings are flat, the Q is high becausethe distributed capacitance is lower than with standard antennas. Inaddition, the current distribution in one winding gets better.

Further, the skin effects are minimized because the wire strip has alarger surface area than cylindrical wires. As is known, the skin effectis an effect where current only flows at the surface of the conductor athigh frequencies instead of through the whole cross section of aconductor as is the case with a DC current.

Another advantage of an air coil using a strip instead of a wire is theimproved fill factor. The fill factor is a measure of the volume ofconductor as a percent of the volume of the entire air coil (i.e.,conductor and insulator). Since the flat strips stack there is lessspace between conductor windings than in a honeycomb arrangement aswould be found in an air coil formed of cylindrical wire. Thisimprovement in fill factor leads to an air coil which is more compact,lighter and thinner.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features of the present invention will be more clearlyunderstood from consideration of the following descriptions inconnection with accompanying drawings in which:

FIG. 1 illustrates a prior art air coil;

FIGS. 2a and 2b illustrate a first embodiment air coil of the presentinvention;

FIG. 3 illustrates a first apparatus for forming an air coil of thepresent invention;

FIG. 4 illustrates a second apparatus for forming the air coil of thepresent invention;

FIGS. 5a-5d illustrate an alternate embodiment coil and the method offorming it;

FIG. 6 illustrates a security badge which may be utilized with thepresent invention;

FIGS. 7a and 7b illustrate a security badge with an air coil presentinvention integrated therewith; and

FIG. 7c illustrates an alternate embodiment security badge which alsoincludes a Wiegand strip.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The making and use of the presently preferred embodiments are discussedbelow in detail. However, it should be appreciated that the presentinvention provides many applicable inventive concepts which can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The following is a description of the air coil of the present invention.The structure of the device will first be described followed by some ofthe advantages it affords. A method of forming the air coil will then bedescribed along with a few of the modifications. An example of one ofthe many specific applications, namely a security badge, which mayutilize the system will then be described.

Referring first to FIGS. 2a and 2b, a preferred embodiment air coil 20is illustrated. The air coil 20 is made from a strip of wire 22 with asubstantially rectangular cross section. In a preferred embodiment, thestrip 22 is an insulated metal ribbon rather than the standard roundwire illustrated in FIG. 1. While copper is the presently preferredmaterial for strip 22, other materials including, but not limited to,aluminum, gold and silver, may be used. As will become clear, magnetwire (i.e., wire coated with a lacquer) may be used. The strip 22 iscoiled up as a spiral resulting in a flat spiral wound coil. In otherwords, the strip 22 is wound in a coil to form a plurality of windings.Insulating material 23 electrically insulates each of the windings fromadjacent ones of the windings.

A coil 20 like that illustrated in FIGS. 2a and 2b has excellent RFproperties in terms of loss factor (the "Q") due to the low distributedcapacitance throughout the coil. This feature is present because eachwinding layer "sees" only two neighbors, namely the previous turn andthe next turn. In a traditional coil 10 (see FIG. 1), this is not thecase. Each winding always sees more.

Also, the loss factor is low because the surface area of the copper ofthe wire conductor 22 is large as compared with a wire with a circularcross section (assuming the same amount of copper for the wire crosssectional area is used). It should be noted that due to the skin effect,the RF current only flows in the wire surface area. Therefore theflatter the wire becomes, i.e., the larger the surface area relative tothe cross sectional area, the more efficient use of copper there willbe.

Another advantage is that the filling factor in the coil is very good.The filling factor is the ratio of the volume of conductor to the totalvolume of the air coil. The fill factor is not as high when circularwire 12 is used in air coil 10 since the circles do not fit perfectlyinto each other. Air gaps between the wire through the coil thereforeresult. These air gaps, on the other hand, are eliminated in the coil 20because the flat ribbon 22 can be compressed without any air gaps sinceflat surfaces abut one another.

A comparison can be made between two coils to demonstrate the advantagesof air coil 20 of the present invention. For example, air coil 20 asillustrated in FIG. 2a and 2b can be compared to a traditional woundcoil 10 as in FIG. 1. The coil 20 can achieve a quality factor of 90(i.e., Q=90) with a coil thickness of only 350 μm. The traditional coil,in comparison, requires a thickness of at least twice that to achievethe same Q. In this context, the Q is defined as the imaginaryresistance divided by the real resistance or mathematically as ##EQU1##Since the frequency ω is given for any system, the Q can be maximized byincreasing the inductance L or reducing the resistance R. Since the wirestrip 22 has a larger surface area for current flow, the resistance R islower for the coil of the present invention.

Two methods of producing the air coil of the present invention will nowbe described. Referring now to FIG. 3, a sheet of conductive foil 30(e.g., copper foil) is used as a base material. The conductive foil 30has an insulating material (not shown) disposed thereon. For example,the insulating material may comprise a lacquer which is coated on oneside of the conductive material 30. This foil is sometimes referred toas "magnet wire." In the preferred embodiment the sheet of conductivematerial 30 is stored on a spool 34.

The foil 30 is rolled off the spool and directed to a cutting device 32.For example, the cutting device 32 may comprise a row of knivesseparated from each other at fixed distances which will simultaneouslycut the foil 30 into ribbons 22a-22n. Although described here is a rowof knives, the cutting device 32 may comprise any and all means forcutting the foil 30 into ribbons 22a-22n. For example, the cuttingdevices 32 may comprise hardened steel or diamond tip material. Anymaterial which is sufficiently hard and will not wear out is desirable.

After the cutting process, each ribbon 22 is wound onto take-up spool35. The ribbon 22 is wound into a spiral coil as described herein above.The insulating material (not shown) will end up between layers of foilthereby electrically insulating each winding from adjacent windings. Inthis manner, there will not be any electrical shorts in the coil afterthe winding process has been completed.

In an embodiment where lacquer is used as the insulating material, thecoils 20 are then heated. This heating step causes the lacquer to meltslightly thereby passivating the coil 22 into a fixed self-supportingcomponent. In one embodiment, the heating step may be performed byapplying hot air (e.g., between about 130° and 170° C.) for betweenabout 5 and 60 seconds. Of course, however, this heating temperature andtime will depend upon the lacquer material used. In an alternateembodiment, the coil can be heated by running a high current through itfor a few seconds.

The coils 20 may then be removed from the take-up spool 35. The wireends may then be pulled out and soldered to the component as required bythe specific application.

In an alternate method, illustrated in FIG. 4, a sheet of conductivefoil 30 is provided. Rather than providing an insulating coating on thesheet 30, an insulation foil 36 is present as an independent material ona separate roll. The two sheets 30 and 36 are compressed at a pinchroller 38 and rolled into each other. The sheets are then cut withcutting device 32 and formed into the rolls as before.

In an alternate embodiment, the insulation ribbons 23 are cut such thatthey are wider than the copper ribbons 30 as illustrated in FIG. 5a. Asa result, the insulation sheet 23 will "hang-over" compared to thecopper sheet 30. When hot air is blown against the plastic overhangafter winding, it will melt on the side, thus passivating the coil 20 toa self supporting structure. A portion of a passivated coil 20 isillustrated in FIG. 5b.

FIG. 5c illustrates a first embodiment apparatus for forming the aircoilillustrated in FIGS. 5a and 5b. Roll 30 of conductive material is fedinto a cutting device 32 which includes wedge shaped blades such asblade 32a. The foil is then separated into ribbons and compressed withinsulating ribbons 23 which are cut from roll 36. The aircoils 22 arethen wound onto a takeup spool 35.

FIG. 5d illustrates a second embodiment apparatus for forming theaircoil illustrated in FIGS. 5a and 5b. In this embodiment, two rolls ofinsulating material 36a and 36b are provided along with a single roll ofconductive material 30. The material on both rolls 36a and 36b are cutinto ribbons of a selected width while the material on roll 30 is cutinto ribbons of a smaller width. Each conductive ribbon 22 is thencompressed with an associated strip 23 of insulating material to formthe aircoils. The aircoils may be stored on takeup spools 35a and 35b.

As discussed above, the air coil of the present invention can be used asa transponder antenna in an identification system such as the onediscussed in U.S. Pat. No. 5,053,774, incorporated herein by reference.One example of the many applications which can utilized theidentification system of the '774 patent, is use with access controlsuch as with a security badge 40 as illustrated in FIG. 6. The air coilof the present invention provides an advantage because it may beintegrated with a security badge 40. One such integrated security badge40 is illustrated in FIG. 7a and in side cross-sectional view in FIG.7b. A transponder chip 42 is electrically coupled to each of the ends44a and 44b of the air coil 20. A detailed description of theidentification system can be found in the '774 patent which isincorporated herein by reference. As examples, the transponder 42 may bedisposed in the center of coil 20 as illustrated in FIG. 7a or in thecorner of the badge card 40 as illustrated in FIG. 7c.

A badge 40 with the coil 20 incorporated therein offers an advantageover other security badges because hand-free access is available. Inother words, a reader can be attached to a locked door allowing the doorto be unlocked without the badge ever being removed from the pocket ofan individual holding it. In addition, as illustrated in FIG. 7c, badgescarrying other access technologies like Wiegand strip 46 or magneticstrip 46 can also carry a transponder simultaneously since there is nointerference between technologies.

To form the security badge 40, the coil 20 is wound between flanges andthen passivated by heat treatment or using a solvent. A transponder chip42 is then soldered to the wire ends and placed in the middle of thecoil. The whole subassembly is then laminated between plastic layers.One advantage the coil of the present invention offers is that it can beused to make laminated plastic smart card where the whole card is notthicker than 0.79 mm (ISO thickness similar to standard credit cards).

While described herein in an application using security badge 40, itshould be noted that the air coil of the present invention can be usedwith identification systems in a great variety of applications. Thetransponder 40 can be attached to or embedded in or simply near anobject such as the security badge 40. This object can be almost anythingimaginable including tires, baggage, laundry, trash containers, keys,vehicles, or even living animals.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of making an air coil comprising thesteps of:providing a sheet of conductive material; disposing aninsulating material adjacent said sheet of conductive material; cuttingsaid sheets of conductive material and insulating material into aplurality of strips; and winding each strip into an air coil such thateach air coil comprises a plurality of windings wherein said insulatingmaterial electrically insulates each of said windings from adjacent onesof said windings.
 2. The method of claim 1 wherein said step ofdisposing an insulating material adjacent said sheet of said conductivematerial comprises coating said conductive material with a lacquer. 3.The method of claim 2 and further comprising the step of heating atleast one of said air coils such that said lacquer melts therebypassivating the coil into a fixed self supporting component.
 4. Themethod of claim 3 wherein at least one of said air coils is heated byblowing hot air on said air coil.
 5. The method of claim 3 wherein atleast one of said air coils is heated by causing a current to flowthrough said coil.
 6. The method of claim 1 wherein said step ofdisposing an insulating material adjacent said sheet of conductivematerial comprises pressing a sheet of insulating material against saidsheet of conductive material.
 7. The method of claim 1 wherein said stepof cutting said sheets of conductive material and insulating materialcomprises cutting said insulating material such that strips of saidinsulating material are wider than strips of said strips of saidconductive material.
 8. The method of claim 7 and further comprising thestep of heating at least one of said air coils such that said strip ofinsulating material of said at least one air coil passivates said atleast one air coil.
 9. The method of claim 1 wherein said step ofproviding a sheet of conductive material comprises providing a sheet ofcopper.
 10. An air coil formed from a method comprising the stepsof:providing a sheet of conductive material; disposing an insulatingmaterial adjacent said sheet of conductive material; cutting said sheetof conductive material and said adjacent insulating material into aplurality of strips; and winding one of said strips into said air coilsuch that said air coil comprises a plurality of windings wherein saidinsulating material electrically insulates each of said windings fromadjacent ones of said windings.
 11. An apparatus for use in making anair coil, said apparatus comprising:a first spool for storing a sheet ofelectrically conductive material; a cutting apparatus for cutting saidsheet of electrically conductive material and an insulating materialinto a plurality of ribbon-like strips; and a winding apparatus forwinding each strip into an air coil such that each air coil comprises aplurality of windings wherein said insulating material electricallyinsulates each of said windings from adjacent ones of said windings. 12.The apparatus of claim 11 wherein said sheet of electrically conductivematerial has a lacquer coating formed thereon such that said insulatingmaterial comprises said lacquer coating.
 13. The apparatus of claim 11and further comprising means for heating each of said air coils.
 14. Theapparatus of claim 11 and further comprising:a second spool for storinga sheet of said insulating material; and a pinch roller for guiding saidsheet of electrically conductive material into a closely adjacentposition with said sheet of insulating material.